The combustion system of a gas turbine generates hot gases. The hot gases can be utilized to drive a turbine. The turbine, in turn, can drive a compressor, wherein the compressor provides compressed air for combustion in the combustion system. Additionally, the turbine produces usable output power, which can be connected directly to power-consuming machinery or to a generator.
The combustion system for a gas turbine may be configured as a circular array of combustion chambers. The combustion chambers are arranged to receive compressed air from the compressor, inject fuel into the compressed air to create a combustion reaction, and generate hot combustion gases thr the turbine. The combustion chambers are generally cylindrically shaped; however, other shapes of combustion chambers are possible. Each combustion chamber comprises one or more fuel nozzles, a combustion zone within the combustion liner, a flow sleeve surrounding and radially spaced from the liner, and a gas transition duct, or transition piece, between the combustion chamber and turbine.
Large gas turbine combustor components have traditionally been fabricated with superalloys, such as, but not limited to, wrought nickel-based superalloys. As turbine designs evolved for operation at higher temperatures, superior low cycle fatigue, oxidation and creep properties of cast superalloys were desired. Also, multiple cast pieces subsequently were joined to turbine combustor components by metallurgical connecting means, such as but not limited to, brazing or welding. However, these means, such as, but not limited to, brazing or welding have not been desirable since the joint locations did not have material properties that match the remainder of the turbine combustor components. Accordingly, a need for turbine combustor components with connected cast pieces is desired where the connected cast pieces have similar material properties as the turbine combustor components as well as the means for connecting the connected cast pieces to the turbine combustor components.
Transition pieces for gas turbine combustors have been formed from various materials, for example, some transition pieces have been formed with a nickel/cobalt based cast alloy, such as GTD-222® (GTD-222 is a registered trademark of General Electric Company, Schenectady, N.Y.). These materials have provided improvement in material properties, such as, but not limited to, at least one of low cycle fatigue (LCF) resistance and creep strength vs. wrought alloys, manufacturability, machinability, weldability, and oxidation resistance, in turbine combustor components including hot gas path parts. These improvements are especially evident with respect to wrought alloy material properties. However, for some high temperature turbine applications, increased material characteristics, such as strength, would provide desirable life potentials of hot gas path parts.
Prior attempts to produce large cast objects with thin walls have not been overly successful. In prior casting attempts problems arose for example, but limited to, when the molten material cools too quickly in the mold due to thinner formed walls, thus resulting in a product that may not have desirable properties for a hot gas path part.
Therefore, a combustion system including a transition piece and method of forming the transition piece that do not suffer from the above drawbacks is desirable in the art.